1) Field of the Invention
This invention relates to air drilling apparatus used typically for drilling through earth formations. Such drilling apparatus is particularly used in forming holes in hard rock, such as granite in mining operations. Such apparatus is also used in drilling water wells in soft or hard rock. In particular the apparatus of the invention relates to a down hole percussion drill, commonly called a pneumatic hammer, rock drill, hammer drill, impact tool, and the like.
2) Description of the Prior Art
Many prior art down hole pneumatic hammers have included an air distributor at the top portion of a casing with a piston adapted to reciprocate between the distributor and a drilling bit placed at the bottom of the casing. Top and bottom finger valves cooperate with the piston to distribute pressurized supply air first beneath the piston and, alternately, above the piston.
One prior art hammer is described in a paper entitled, "Hammer Drill Reduces Air Drilling Costs" by Walter E. Liljestrand of Mission Manufacturing Company, Houston, Tex., presented to the 19th Annual Meeting of the American Associates of Oilwell Drilling Contractors, Oct. 13, 1959 at Oklahoma City, Okla. Such hammer has a casing with a piston having a top portion and a lower portion. The upper portion of the piston is smaller in diameter than is the lower portion of the piston. Supply air is channeled in an annulus about a relief tube through the center of the piston. For a given outside diameter of the casing, the top end of the top portion of the piston is limited in area due to the reduced diameter of the upper portion of the piston. Accordingly, the performance of such hammer, for a given level of pressure of the pressurized air supply entering the hammer, is limited.
Another prior art hammer is described in U.S. Pat. No. 4,084,646 issued Apr. 18, 1978 and assigned to Ingersoll-Rand Company. Such hammer includes a top sleeve disposed between the drill casing and the piston to channel supply air from a distributor at the top of the hammer to the bottom end of the piston. The Ingersoll-Rand hammer, like the Mission hammer, includes both top and bottom finger valves, and requires a reduced diameter of the upper part of the piston to slide within the top distributing sleeve. Again, the smaller surface of the top end of the piston, for a given outer diameter of the casing and for a given air supply pressure, prevents the hammer from operating at optimum performance.
Other features of the prior art hammers described above have contributed to less than ideal performance. For example, a choke placed in an axial passage way of the Ingersoll-Rand hammer feeds a portion of the bypassed air pressure periodically to a position above the top end of the piston. Such bypassed air pressure increases the back pressure of the drill and decreases its performance somewhat.
Another example concern the operation of the hammers when they are lifted from the bottom of the hole. Such hammers are constructed such that continuing reciprocation of the hammer may continue with possible damage to the piston.